Directional drilling is commonly accomplished by use of a progressive cavity motor that is located beneath the surface and driven by high pressure fluid or "mud".
Progressive cavity motors typically have a single shaft in the shape of one or more helices contained within the cavity of a flexibly-lined housing. The lined cavity is in the shape of two or more helices (one more helix than the shaft) with twice the pitch length of the shaft helix. Either the shaft or the housing is secured to prevent rotation; the part remaining unsecured rolls with respect to the secured part. In so rolling, the shaft and housing form a series of sealed cavities which are 180 apart. As one cavity increases in volume, its counterpart cavity decreases in volume exactly at the same rate. The sum of the two volumes is therefore constant. By pumping high pressure drilling fluid or "mud" into the wall casing and through the progressive cavity motor, the rotor can be caused to rotate so as to cause a progression of cavities which eventually allows the fluid to exit the progressive cavity device.
When used for directional drilling, the pump rotor is connected by a flexible coupling to a drill bit drive shaft. The drill bit drive shaft, in turn, drives the drill bit to effect drilling.
Radial and thrust bearings are typically located along the drive shaft to properly position the drive shaft and to react (absorb) radial and vertical loads. A flow restricter is also located along the drive shaft to direct the mud exiting the progressive cavity motor into an internal drive shaft passage and ultimately through the nozzles in the drill bit to flush drilling debris away from the drill bit and carry the debris back to the surface. The flow restrictors may be positioned anywhere along the drive shaft; however, they should operate to minimize flow through the bearing cavity.
In most drilling motors, the bearings are lubricated by the drilling mud. However, the mud is typically somewhat abrasive; consequently, its use as a lubricant causes the bearings to wear relatively quickly requiring costly changes of bearings in the field. A sealed oiled lubricated bearing assembly would greatly extend bearing life.
There have been several attempts at sealing an oil filled bearing system. The primary approach has been to equalize pressure across the seals such that the seals operate to separate the oil from the mud rather than also sealing against differential pressures. U.S. Pat. No. 4,593,774 to Lingafelter discloses a down hole bearing assembly which includes a two volume, three seal arrangement that allows the inner seal to be lubricated by oil and react the pressure differential. The outer two seals have no differential pressure and seal against abrasives only.
U.S. Pat. No. 4,577,704 to Auman discloses a bearing system for a down hole motor which uses a flow passage from the high pressure internal passage to the low pressure side of the bearing to eliminate the pressure across these seals. This communication path could cause catastrophic results if the passage increased sufficiently to eliminate the pressure drop across the bit.
While prior art systems such as those mentioned above, have met moderate success none of them have been entirely successful. Among other things, they do not take into account all operating conditions down hole. As a result, known sealed bearing constructions typically include one or more design features which have proven to be disadvantageous. In a number of known constructions, notably the Auman patent discussed above, fluid flow paths are provided between the internal drive shaft passage (high pressure side) and the exterior of the drive shaft after the drill bit nozzles (the low pressure side). As fluid flows through these passages, the path can become eroded and mud flow to the drill bit can be cut off causing failure of the drill bit. Other known constructions fail to adequately accommodate drive shaft deflections caused by large side loads on the drill bit. In such cases, significant shaft deflection can cause seal run out and early failure. Further, many known pressure equalization systems which, according to design, should not allow leakage of mud into the bearing assembly, in practice have been found to under certain conditions, permit mud to enter the sealed assembly. A number of known assemblies include elements in sliding contact with one another. Naturally, these constructions tend to wear relatively quickly. Additionally, in some known constructions the seal is not properly located and restrained in both the radial and longitudinal directions and as a result, the faces of the seal separate. Also, many known constructions do not include a backup seal or an exclusion seal. Another problem experienced in downhole drilling arises from the occasional need to replace bearings in the field. It can easily be appreciated that changing a bearing assembly which operates hundreds or thousands of feet below the surface is always complicated and expensive. Such field changes are further complicated by the complex multi-part construction of many known bearing assemblies. While a long lasting bearing assembly would reduce the frequency of such field changes they would still be necessary from time to time. Thus, for sometime, there has been a need for an oil filled bearing system that can be readily field changed.